A rangefinder is a device capable of measuring the line-of-sight distance to a remote object. Rangefinders may employ various types of technology, for example based on laser, radar or sonar. In a typical laser rangefinder using a time-of-flight technique, the rangefinder transmits a laser pulse towards a target object, and the time taken for the pulse to be reflected off the object and travel back to a detector of the rangefinder is used to compute the distance to the object. Typically, a rangefinder transmits a sequence of laser pulses periodically at a relatively high frequency (e.g. 200 Hz) to obtain periodic samples of the distance. The samples obtained over a certain time window may be filtered, for example by averaging, to obtain a measured distance.
One problem with conventional laser rangefinders is that, due to movement of the operator and/or movement of the target object, it may be difficult for the operator to keep the laser beam directed exactly at the target object. This may result in null data, for example if the beam is pointed at a location (e.g. the sky) where the beam is not reflected back to the rangefinder. It may also result in spurious data, for example if the beam is pointed at an object (e.g. a foreground or background object) other than the target object. Consequently, any measured distance may be unreliable. This difficulty may be increased when the target object is a relatively large distance away and/or if the target object is moving relatively quickly.
Furthermore, it may be difficult for the operator to know whether the target object has been validly acquired, that is whether the beam is directed at the target object in a sufficiently accurate and stable manner so as to achieve a reliable distance measurement. If the target object is moving, it may be difficult for the operator to know whether the target object is being validly tracked, that is whether the beam is moved so as to follow the target object in a sufficiently accurate and stable manner so as to achieve continuous distance measurements.
What is desired is a technique for obtaining more reliable distance measurements using a rangefinder. What is also desired is a technique for determining when a target object has been validly acquired. What is also desired is a technique for determining when a target object is being validly tracked.